1. Field of the Invention
The invention relates generally to an engine system and a method for controlling the same, and, more specifically, to an engine system provided with a variable valve timing system and a method for controlling the same.
2. Description of the Related Art
A variable valve timing (VVT) system that changes the phase (i.e., crank angle), at which an intake valve or an exhaust valve is opened/closed, based on the engine operating state has been used. Such variable valve timing system changes the phase of the intake valve or the exhaust valve by rotating a camshaft, which opens/closes the intake valve or the exhaust valve, relative to, for example, a sprocket. The camshaft is rotated hydraulically or by means of an actuator, for example, an electric motor.
With a variable valve timing system that hydraulically drives a camshaft, the variable valve timing control is sometimes not executed as accurately as it should be, in a cold environment or at the time of engine starting. Such inconvenience is caused because the hydraulic pressure used to drive the camshaft is insufficient or the response of the camshaft to the hydraulic control is slow in such occasions. To obviate such inconveniences, a variable valve timing system that drives a camshaft by means of an electric motor has been suggested, as described in, for example, Japanese Patent Application Publication No. JP-2005-98142 (JP-A-2005-98142), Japanese Patent Application Publication No. JP-2005-48707 (JP-A-2005-48707), and Japanese Patent Application Publication No. 2004-156461 (JP-A-2004-156461).
JP-A-2005-98142 and JP-A-2005-48707 each describe a variable valve timing system that changes the rotational phase of a camshaft relative to a crankshaft, namely, the valve timing in accordance with the rotational phase difference between a sprocket and a guide rotating body that is rotated by an electric motor. JP-A-2005-98142 describes a mechanism that changes the ratio of the amount of change in the rotational phase of a camshaft relative to a crankshaft (valve timing), with respect to the amount of change in the rotational phase of a guide rotating body relative to a sprocket depending on the phase region in which the phase of the valve timing is present. As shown in FIG. 16 in JP-A-2005-98142, the above-described change-amount ratio is relatively low in the phase region in which the valve timing is delayed, while the above-described change-amount ratio is relatively high in the phase region in which the valve timing is advanced.
With the configuration described in each of JP-A-2005-98142 and JP-A-2005-48707, the valve timing is changed by reducing the relative rotational speed between the output shaft of the electric motor that rotates the guide rotating body and the sprocket at the speed reduction ratio corresponding to the above-described change-amount ratio. Namely, with the configuration described in JP-A-2005-98142, the speed reduction ratio is variably set based on the phase region in which the phase of the valve timing is present.
Accordingly, in the phase region in which the speed reduction ratio is low, that is, the phase region in which the amount of change in the valve timing with respect to the relative rotational speed between the output shaft of the electric motor and the sprocket is large, the valve timing may be undesirably changed due to the rotation of the output shaft of the electric motor, which is caused by a reaction force applied to the camshaft, when the engine is stopped.
Especially, when the variable valve timing system described in JP-A-2005-98142 is mounted in a vehicle (for example, a hybrid vehicle) provided with a speed reduction control mechanism that reduces the engine speed in a predetermined speed reduction pattern by using, for example, a motor torque in order to dampen vibration of the vehicle when the engine is stopped, the configuration needs to be such that an undesirable change in the valve timing is not caused when the engine is stopped.